A laser harp consists of seven beams of light, each for a natural tone. When a beam is blocked a sound is played, as simple as it is. Well, there are more details about it but that's basically how it works.
Honestly, there's not much you can do with just seven plain notes, except for playing simple melodies, the same kind of melodies you could play with a Casio PT-87, or even simpler tunes. But, what the hell, the whole spectacle of movements, lights and sounds was impressive. In addition, to see the beams of light it's necessary to fill the air with smoke, which creates an even more intimate atmosphere.
After the concert, me and Adrian were commenting on how fun was the laser harp, and joking if it would possible to make our own. As usually happens, it seems someone though about it before.
Stephen Hobley, a music & electronic hobbyist, manage to make his own laser harp. Some parts are not public, and a purchase for the plans is required, but there's still some videos where he explains how to build your own laser harp.
He even improved Jarre's laser harp by using a Wiimote to detect the height of hands. By moving your hand up and down, this laser harp can vary the pitch of the sound playing.
I love him playing Rendez-vous II.
Last but not least, check these guys from Mountain Glen Harps who handmade truly laser harps, or rather say, harps consisting of lasers beams instead of strings. The funny thing is that since there's not smoke in the air, they look more like doing air-harp playing :)